The invention relates to an arrangement in a mechanical shaft seal and more particularly to an arrangement in a mechanical shaft seal comprising at least one sliding surface part rotating with a shaft in relation to a frame of the device, at least one sliding surface part fastened to the frame and/or to a separate frame part that is non-rotatable in relation thereto, the sliding surface part rotating in relation to the frame and the non-rotating sliding surface part are provided with sliding surfaces pressed against one another, at least one additional part arranged to connect the sliding surface part rotating in relation to the frame to the shaft and/or to at least one insertion part fastened to the shaft and rotating therewith in order to transfer the rotating motion from the shaft to the sliding surface part, and at least one additional part arranged to connect the sliding surface part, which is non-rotatable in relation to the frame, to the frame or at least to one insertion part connected to the frame in order to prevent the rotation of the sliding surface part in relation to the frame.
Known mechanical shaft seals typically comprise at least one rotating sliding surface part, which is fastened to a shaft rotating in relation to the frame of the device and at least one non-rotatable sliding surface part fastened to the frame of the device or to a separate frame part. These shaft seals also comprise separate additional parts preventing or shifting the rotation in relation to the frame, at least one spring ensuring the contact of the sliding surfaces to one another during operation, at least one insertion part that allows directing the spring forces to other parts. In addition, the shaft seal may comprise different auxiliary seals, which are used to seal the gaps between the parts mentioned above.
During operation the sliding surfaces of the sliding surface parts in the mechanical shaft seal pressed against each other rotate against one another, while one sliding surface part rotates with the shaft of the device and while the other sliding surface part remains non-rotatably fastened to the device or to the separate frame part. The purpose of these sliding surfaces is a gap formed between the shaft rotating in relation to the frame of the device and the frame. When operating, frictional force is exerted between the plane surfaces of the parts in the mechanical shaft seal rotating against one another, the frictional force being caused when the parts come into contact with one another. Depending on the prevailing conditions a medium inside the device or a medium to be conducted inside the mechanical shaft seal especially intended for this purpose cools and lubricates the plane surfaces of the sliding surface parts in the mechanical shaft seal. The purpose of the medium is to penetrate between the rotating plane surfaces and to reduce the frictional force exerted between the plane surfaces and thus to reduce the creation of thermal energy caused by the frictional force on the plane surfaces of the sliding surface parts in the mechanical shaft seal.
The rotating sliding surface part of such a mechanical shaft seal is connected to the shaft of the device by means of the additional parts included in the shaft seal and designed for this purpose or by means of machining features forwarding the torque caused by the rotation of the shaft to the rotating sliding surface part of the mechanical shaft seal, thus achieving the rotation of this part with the shaft. Correspondingly the non-rotating additional parts are connected to the device or to a separate frame part by means of the parts included in the mechanical shaft seal and designed for this purpose or by means of machining features preventing the non-rotating sliding surface from rotating in relation to the frame or to a separate frame part by impact of torque conveyed through the frictional force directed thereto by the sliding surface part rotating with the shaft.
The problem with the above arrangement is that the additional parts transferring torque from the shaft of the device included in the shaft seal or the machining features are subjected to wear or are broken at the points, from which torque is transferred from one part to another, for instance to the sliding surface parts. The same problem is known as regards the parts intended to be non-rotating at points, from which the non-rotating parts are locked to the device or to a separate frame part. The torque caused by frictional force formed between the plane surfaces of the non-rotating parts and the rotating parts in the mechanical shaft seal wears and breaks the additional parts or the machining features, by means of which the rotating motion of the shaft of the device is transferred to the rotating parts of the mechanical seal, or which tend to be used for preventing the rotating motion produced by the torque caused by the frictional force in the non-rotating parts of the mechanical shaft seal. This phenomenon causes the mechanical shaft seal to be prematurely damaged in such a manner that the mechanical seal no longer operates as planned for sealing the gap between the rotating shaft and the static parts of the device. In addition, the torque causes the sliding surfaces of the sliding surface parts in the mechanical seal to deform so that the mechanical seal no longer operates as planned.